Have you ever quickly needed to see how much traffic a host is sending/receiving or how much traffic is in a flow between hosts. IP accounting can easily provide the amount of packets and data for each source/destination pair.
By enabling the following under the interface:
R4#interface fa0/0
ip accounting output-packets
After a couple minutes, you should get a quick idea of the data flows
R4#sh ip accounting output-packets
Source Destination Packets Bytes
192.168.89.171 192.168.54.72 2394 1410785
192.168.89.171 192.168.54.10 5119 6976939
192.168.89.239 192.168.54.10 662 194296
192.168.89.211 192.168.54.155 124 15048
Routing-Bits 
This sums up the technology very well. I will like to add that we can define filters to specify which hosts that IP accounting information is kept. We can use the ip accounting-list global configuration command:
ip accounting-list ip-address wildcard
yes that command has been very handy, thanks :)